1. Technical Field of the Invention
This invention relates to the purification of propylene oxide. More particularly, this invention relates to a distillation process for removing contaminating quantities of impurities, including oxygen-containing impurities such as methanol, acetone and water from an impure propylene oxide feedstock. Still more particularly, this invention relates to a method wherein an impure propylene oxide feedstock containing oxygen-containing impurities, for example, from about 50 to 4,000 ppm of methanol, from about 0.01 to about 2 wt. % of water, and from about 0.01 to about 2 wt. % of acetone is purified in an extractive distillation column using 1-propanol as the extractive distillation agent.
2. Prior Art
It is known to react a hydroperoxide feedstock such as tertiary butyl hydroperoxide with propylene in the presence of an epoxidation catalyst in order to provide a reaction product comprising propylene oxide, an alcohol corresponding to the hydroperoxide feedstock, a solvent, and impurities (see, for example, Kollar U.S. Pat. No. 3,350,422, Kollar U.S. Pat. No. 3,351,635 and Sorgenti U.S. Pat. No. 3,666,777.
It is also known to separate the reaction product by distillation in order to obtain a plurality of fractions including, for example, a propylene recycle fraction, a propylene oxide product fraction, an alcohol fraction, etc.
It is also known that methanol, acetone and water are common contaminants for propylene oxide which are removed only with difficulty.
Washall U.S. Pat. No. 3,578,568 discloses a process for removing oxygen-containing impurities such as acetone, acetaldehyde and methanol from impure propylene oxide using a glycol such as ethylene glycol or propylene glycol as an extractive distillation agent.
Hoorl and Newman U.S. Pat. No. 3,632,482 is directed to a propylene oxide recovery process by extractive distillation using an alcohol-ketone-hydrocarbon solvent. The invention relates to a method for the purification of crude propylene oxide contained in a mixture produced by the epoxidation of propylene with an organic hydroperoxide and calls for extractive distillation of the crude propylene oxide in a plurality of successive extractive distillation zones with the aid of a solvent mixture consisting essentially of hydrocarbons, alcohols, and/or ketones corresponding to the organic hydroperoxide employed in producing the propylene oxide. In the preferred embodiment of their invention, the extractive distillation agent is a recycle fraction from a three column distillation sequence wherein the bottoms from the third distillation column are flashed to obtain an overhead composed of hydrocarbons, alcohols and/or ketones which is recycled as an extractive distillation agent to the three distillation columns involved in the propylene oxide purification sequence.
Burns et al. U.S. Pat. No. 3,715,284 discloses a process for the purification of impure propylene oxide using acetone or a mixture of acetone with methanol which is introduced into a distillation column either below or together with the impure propylene oxide.
Schmidt U.S. Pat. No. 4,140,588 discloses a process for the purification of propylene oxide contaminated with methanol and acetone using water as an extractive distillation agent, the water being introduced into the distillation column above the point of introduction of the crude propylene oxide feed.
Schmidt states at column 2, lines 50-55 that: "Propylene oxide, however, has a substantial solubility in water and is readily hydrolyzed to propylene glycol (PG) in the presence of large amounts of water"--i.e., in the reboiler section of the tower.
U.S. Pat. No. 3,578,568 discloses the use of glycols or glycol ethers in an extractive distillation to remove oxygen containing impurities such as acetone, acetaldehyde, and methanol. It is claimed that the concentration of the solvent in the vapor space in the extractive distillation zone of the distillation tower is preferably between 15 and 50 mole percent of the total vapor.
Shih et al. U.S. Pat. No. 5,000,825 discloses the purification of monoepoxides such as propylene oxide that are contaminated with oxygenated impurities such as water, low molecular weight alcohols, low molecular weight ketones, low molecular weigh aldehydes and the like by the extractive distillation of the contaminated monoepoxide using a lower glycol containing 2 to 4 carbon atoms. Examples of lower glycols that are given in the patent include ethylene glycol, 1,2-propane diol, 1,3-propane diol, 1,4-butane diol, 1,2-butane diol, 1,3-butane diol and 2,3-butane diol. It is stated that higher diols or higher glycol ethers do not provide sufficient selectivity for the removal of such impurities and are not included as the extractive distillation solvents suitable for use in the invention.
Meyer et al. U.S. Pat. No. 4,971,661 discloses the use of an aqueous acetone extraction to remove methanol from propylene oxide.
Meyer et al. point out that the presence of additional acetone (added to feed or solvent) serves as a buffer between the reboiler section and the balance of the tower. This is apparent if one looks at the normal boiling points (i.e., atmospheric pressure):
______________________________________ Component NBP (.degree.C.) ______________________________________ Propylene Oxide (PO) 34 Acetone 56 Water 100 ______________________________________
The acetone serves as a buffer section in the tower between the PO and water (a high concentration of water is in the reboiler and a high concentration of PO is above the acetone buffer zone). The acetone buffer zone limits the contact of PO with a high concentration of water. It is apparent that the additional acetone makes its presence known in the reboiler as well as evidenced by lower reboiler temperatures. This also helps reduce PO to PG conversion as the reaction rate increases with increasing temperature. Any PO making its way to the reboiler will see a lower temperature, thus reducing its conversion to PG.
It is clear that the tower should be operated at as low a pressure as is practical to minimize PO loss.
Shih et al. U.S. Pat. No. 5,006,206 discloses a method for the separation of propylene oxide from hydrocarbon impurities using a mixture of t-butyl alcohol and water as an extractive distillation solvent.
In U.S. Pat. No. 5,129,996, Shih discloses a method for the separation of propylene oxide from hydrocarbon impurities using a glycol having 2 to 5 carbon atoms as an extractive distillation solvent.
Peters et al. U.S. Pat. No. 5,464,505 discloses a method for the separation of propylene oxide from impurities such as water, methanol and acetone using, as the extractive distillation solvent, propylene oxide adducts of oxyethylene glycols. The propylene oxide adducts of oxyethylene glycols form in situ in the extractive distillation tower by the reaction of propylene oxide with oxyethylene glycol extractive distillation agents.
As shown by the foregoing discussion of the prior art, the results heretofore obtained in the purification of propylene oxide by extractive distillation have not been entirely satisfactory, particularly in respect of the loss of propylene oxide by reaction with water, extractive distillation agents, etc., in the distillation tower.